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MG MGB Technical - Battery drains on its own

I have a 1980 B. A few days back the starter was sluggish, then would not turn over at all. I put in a new battery (Interstate) because the old one was five years old (the car is not driven in the winter and I put a trickler charge on it) and the next couple of days, no problem. then the starter would barely turn, you know that sluggish sound that suggests the battery is dying. But the clock, interior lights and the open door buzzer still worked, suggesting there was juice left still. I did not jump or push start it, and let it sit for a couple of days, and now the battery is totally dead, with no juice. Something caused the battery to drain. Since I was able to jump start it, and the juice seemed to continue to drain from the battery, I am assuming the alternator and starter are not involved in this problem. Any ideas as to why the battery drains flat on its own?
Dan Beyer

A bad alternator can drain the battery - so can a light left on (boot/trunk?).

Disconnect the battery main lead and connect a low wattage 12V bulb between the battery and the lead - it should glow if there is a drain.

Just unplug stuff until the light goes out to find the culprit.
Chris Betson

I had the same probem awhile back. For security reasons, I installed a battery shutoff switch on the positive side at the battery post. I only turn it on when i am using the car. This is not a fix but it has gotten me by for the last 2 years. If you must drive the car, I suggest disconnecting the battery when not in use so you can have enough charge when you need it. I also suggest that you take it to a shop that can test your charging system.
kids1

How bright are the headlights? Bad bearings in the starter can cause the starter to bind and require extra energy to turn.
Kimberly

In my case, most of my trips are only about a mile --- not a good distance for any car, and certainly not enough to repay the battery for the starting load.

I purchased a trickle charger from Sears for $20-$30 that keeps my battery topped off. Even if the car is cold from two weeks of sitting, it starts right off.
Dan

As Chris mentions it's probably something left on or it could very well be a bad alternator. If the alternator is not charging fully it would cause this but one would assume the battery warning light would alert you of such a condition. Another possibility is a bad diode in the alternator that allows the current to flow back though the alternator when the engine is not running, thus draining the battery. That happened to a fiend of mine, he was pulling his hair out because he disconnected every accessory from the wiring harness and still had a draw from the battery. Not until the alternator was disconected did the draw dissapear.

-Jared
Jared Snider

Depending on how small the drain is it may not light even a low wattage 12v bulb wired in place of the battery ground strap. I use an analogue voltmeter on its 12v scale. You should see a few volts registered on an alternator equipped car, but that is the normal reverse leakage of the diodes and can be ignored. If you see a full 12v then you do have a drain, and unplugging the alt, then the purple fuse, are the first things to do.
Paul Hunt

Don't you mean an Ammeter and a few milliampers of reverse leakage for the recrtifier diodes? Across what are you measuring voltage to determine leakage current?

Does anyone know the "normal" amount of leakage current for an "off" MG with no radio, clock, etc?

Matthew Jones

No, voltmeter. An ammeter is a very low resistance instrument which will attempt to pass whatever current is flowing when the battery ground cable is connected, which may be many amps, which may be more than the safe rating of the instrument and leads you are using and so damage it/them. A voltmeter is a very high resistance instrument which limits any current that can flow to microamps. This is why you can safely connect a voltmeter to the terminals of a battery, but only the foolish or unknowing would do that with an ammeter.

With a voltmeter connected in place of the ground strap you are measuring the voltage across a series circuit that consists of the battery and any component that is connected between the battery hot post and the body ground and is leaking battery current, which can be represented as a resistor. No leakage, no connection between the battery hot post and the body ground, so no voltage measured by the meter. With battery leakage problems the 'resistance' is a tiny fraction of that of the voltmeter, so very little of the battery voltage is dropped across the leakage resistance and the majority across the voltmeter hence it displays battery voltage. With the normal reverse leakage current of the alternator diodes this 'resistance' is of a very high value, so a significant proportion of the battery voltage is dropped acroiss it, leaving less for the voltmeter which displays a value less than battery voltage.

A car with a dynamo should show no leakage current at all. With an good alternator there will be a few microamps leakage.
Paul Hunt

My ammeter will go up to 10 amperes, so unless I have a MAJOR problem, I will not damage it (unless I am tired and connect it in paralell with the battery.) I also have some nice, precision low resistance shunts to use if I need to measure a high current such as the starter motor current. I'm neither foolish nor stupid, but I'll admit to ignorance or carelessness when necessary. I am gratefull to the BUSS company for their fine line of fuses, some of which have saved me from meter destruction!

If there were a "perfect" voltmeter with infinite input impedance, one would, of course, only measure the voltage of the battery, as the meter impedance would be greater than the resistance of the car.

But, (and here's the part I didn't get) you said you are using an ANALOG meter, with a much lower input impedance - enough to draw current and cause some voltage drop accross the "leaky" car, and thus give a usefull reading.

Very clever - sorry I didn't catch that - and usefull if one does not happen to have an ammeter capable of reading the currents involved. (plus, as you said, it's safer)

Thanks for the insight.

Matthew Jones

This thread was discussed between 04/07/2003 and 11/07/2003

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